1. Field of the Invention
The present invention relates to a flow rate sensor for measuring an amount of intake air in an internal combustion engine, for example, and relates particularly to a thermosensitive flow rate detecting element provided with a heating element for measuring a flow velocity or flow rate of a fluid based on heat transfer to the fluid from the heating element or a portion heated by the heating element, and further relates to a method for the manufacture thereof.
2. Description of the Related Art
FIG. 14 is a front elevation showing a first conventional thermosensitive flow rate detecting element such as described in Japanese Patent Laid-Open No. HEI 6-249693, for example, and FIG. 15 is a cross section taken along line XV—XV in FIG. 14 viewed from the direction of the arrows.
In FIGS. 14 and 15, first and second films 63 and 64 are formed separately on a front surface of a silicon substrate 67 made by a single-crystal silicon. A measuring element 61 is formed on the first film 63, and a medium temperature measuring element 62 is formed on the second film 64. A frame 66 is prepared by forming two notches 69 having a trapezoidal cross-sectional shape in a rear surface of the silicon substrate 67. These notches 69 are formed so as to reach the first and second films 63 and 64. Thus, the first and second films 63 and 64 are stretched across the frame 66, forming a diaphragm construction, and a silicon support 68 functioning as an isothermal element is disposed between the first and second films 63 and 64. In addition, electrode terminals 65 connected to the measuring element 61 and the medium temperature measuring element 62, respectively, are formed on the front surface of the frame 66.
Next, operation of a first conventional thermosensitive flow rate detecting element 60 constructed in this manner will be explained.
A mass flow such as an airflow, for example, is allowed to flow over the front surface of the first thermosensitive flow rate detecting element 60 as indicated by an arrow A in FIG. 14. Then, the measuring element 61 is heated by applying an electric current to the measuring element 61 by means of the electrode terminals 65. Electrical resistance in the measuring element 61 is also measured. Moreover, the measuring element 61 is designed such that the resistance changes with temperature. The measuring element 61 is cooled by the passing airflow. The extent of this cooling is dependent upon the mass flow of the passing medium. Thus, when the current heating the measuring element 61 is maintained constant, the strength of the flow of the medium can be detected by measuring the resistance in the measuring element 61.
Moreover, the medium temperature measuring element 62 is used in order to suppress the influence of the medium on the measurement signal.
In this first conventional thermosensitive flow rate detecting element 60, because the notches 69 are formed by etching, there is a certain amount of irregularity in the size of the diaphragm portions. And because the diaphragm portion of the first film 63 contacts the frame 66 around the entire perimeter, some of the heat generated by the measuring element 61 is not cooled by the airflow, but is instead transferred through the diaphragm portion to the frame 66. Thus, irregularities in the size of the diaphragm portion lead to irregularities in the amount of heat transferred through the diaphragm portion to the frame 66. As a result, one problem has been that flow rate detection characteristics differ among first thermosensitive flow rate detecting elements 60, preventing accurate flow rate detection.
Furthermore, the medium temperature measuring element 62 is formed on the diaphragm portion of the second film 64 in order to ensure responsiveness to temperature changes in the medium. However, because the diaphragm portion of this second film 64 contacts the frame 66 around its entire perimeter, it is easily subjected to the influence of the temperature of the frame 66. Thus, one problem has been that irregularities arise in medium temperature detection performance as a result of the irregularities in the size of the diaphragm portion.
In order to solve these problems, a thermosensitive flow rate detecting element is proposed in Japanese Patent Laid-Open No. HEI 9-43018, for example, in which a heat transfer layer is formed in the heat transfer pathway by which the heat generated by the heating body (the measuring element 61) passes through the diaphragm portion and is transferred to the substrate (the frame 66), the heat generated by the heating body being transferred to the substrate through the heat transfer layer.
FIG. 16 is a front elevation showing a second conventional thermosensitive flow rate detecting element such as described in Japanese Patent Laid-Open No. HEI 9-43018, for example, and FIG. 17 is a cross section taken along line XVII—XVII in FIG. 16 viewed from the direction of the arrows.
In FIGS. 16 and 17, a diaphragm layer 77 is formed on a front surface of a substrate 71, and notches 78 having a trapezoidal cross-sectional shape are formed so as to reach the diaphragm layer 77 from a rear surface of the substrate 71. Thus, the diaphragm layer 77 is stretched across the substrate 71, constituting a diaphragm portion 72. A heating body 73 and temperature sensors 74 are formed on the diaphragm portion 72. In addition, heat transfer layers 75 made by metal layers are formed so as to cover edge portions of the diaphragm portion 72 on first and second sides of the heating body 73 and the temperature sensors 74. Furthermore, electrode terminals 76 connected to the heating body 73 and the temperature sensors 74, respectively, are formed on the front surface of the substrate 71. In addition, a protective layer 79 is formed by coating so as to cover the heating body 73, the temperature sensors 74, the heat transfer layers 75, etc.
In a second conventional thermosensitive flow rate detecting element 70 constructed in this manner, the heating body 73, the temperature sensors 74, and the heat transfer layers 75 can be formed simultaneously by forming a thermosensitive resistor film on the diaphragm layer 77 then using photoengraving techniques and etching techniques. Thus, relative positional relationships among the heating body 73, the temperature sensors 74, and the heat transfer layers 75 are ensured with high precision. The coefficient of thermal conductivity of the heat transfer layers 75 is extremely large compared to that of the diaphragm layer 77. The heat transfer layers 75 are formed so as to cover two facing edge portions of the diaphragm portion 72. Consequently, the heat generated by the heating body 73 is transferred to the heat transfer layers 75 through the diaphragm portion 72, and then transferred to the substrate 71 through the heat transfer layers 75. Thus, because the amount of heat transferred from the heating body 73 to the substrate 71 is constant regardless of the size of the diaphragm portion 72, it is claimed that irregularities in the flow rate detection characteristics between first thermosensitive flow rate detecting elements 60 are suppressed, enabling accurate flow rate detection.
In an internal combustion engine in an automobile, vibrations of 40 to 50 g (1 g=approx. 9.8 m/s2) are generated, and there are also cases where the flow velocity of the intake air reaches 200 m/s or more. Furthermore, when a backfire occurs, a pressure of nearly two atmospheres may also act on the thermosensitive flow rate detecting element. Thus, when a thermosensitive flow rate detecting element having a diaphragm construction is subjected to mechanical stress of this kind, if the size of the diaphragm portion is larger than a design value (a desired value), or if there is an abnormality in the shape of the diaphragm portion, problems arise such as the diaphragm portion being damaged.
However, in the first conventional thermosensitive flow rate detecting element 60, because the diaphragm portion is formed by etching the silicon substrate 67, there is a certain amount of the irregularity in the size of the diaphragm portion. As a result, in the first thermosensitive flow rate detecting element 60, irregularities arise in the flow rate detection characteristics and the fluid temperature detection performance, preventing accurate flow rate detection, and if applied to uses in measuring the amount of intake air rate in an internal combustion engine of an automobile, problems may arise such as the occurrence of accidents which are damaging to the first thermosensitive flow rate detecting element 60.
On the other hand, in the second conventional thermosensitive flow rate detecting element 70, although irregularities in the flow rate detection characteristics and the fluid temperature detection performance resulting from irregularities in the size of the diaphragm portion are suppressed by forming the heat transfer layers 75, no consideration is given to keeping the size of the diaphragm portion 72 to a design value, and if applied to use in measuring the amount of intake air rate in an internal combustion engine of an automobile, problems may arise such as the occurrence of accidents which are damaging to the second thermosensitive flow rate detecting element 70 as a result of the irregularities in the size of the diaphragm portion.